1. Field of the Invention
The present invention relates to a synchronous semiconductor memory device and a method of testing the same, and more specifically, it relates to an input/output circuit inputting/outputting data in synchronization with a clock, a synchronous semiconductor memory device including the same and a method of testing the same.
2. Description of the Prior Art
In general, a data input/output circuit employed for a semiconductor device such as a semiconductor memory device, for example, puts a plurality of outputted data out of phase with an internal clock thereby transferring the data from the semiconductor memory device to an external device at a rate higher than the clock frequency.
In order to reduce a test cost, a BIST (built-in self test) structure providing a function of performing read and write tests of memory cells on a chip itself is increasingly employed.
FIG. 67 is a block diagram showing the block structure of a conventional memory having a BIST (built-in self test) function.
Referring to FIG. 67, this memory includes a clock generation circuit CKG generating an internal operation clock in response to control signals /RAS, /CAS and /WE, an address buffer ADB externally receiving an address signal AI, an X decoder XDEC and a Y decoder YDEC decoding the address signal in accordance with the clock generated by the clock generation circuit CKG, and a memory cell array MA transmitting/receiving data to/from an external device.
This memory further includes a self test circuit STC. The self test circuit STC includes a ROM storing a coded test procedure, a program counter PC, an address counter CA, a data generation circuit DG, a data compare circuit DC and a test clock generation circuit TCG.
In order to read a desired instruction from the ROM in a self test, the program counter PC specifies an address of the ROM storing the instruction. The ROM sequentially outputs the coded test procedure for controlling the program counter PC, the address counter AC, the data generation circuit DG, the data compare circuit DC and the test clock generation circuit TCG and progressing a memory test.
When such a self test circuit is built in the semiconductor memory device, a high-performance test can be executed with a simple tester, to reduce the test cost.
When interleaving a plurality of data in a data input/output circuit following the recent increase of the operating speed of the semiconductor device, however, the data may collide with each other when picked up by an externally connected circuit, or erroneous data may be picked up.
In a gigantic synchronous semiconductor memory device having a memory capacity reaching 1 Gbit, a skew of an internal signal, particularly a clock controlling the overall operation of the chip increases to limit the operating frequency of the chip. Particularly when receiving an externally inputted reference clock in a clock buffer and thereafter receiving an address, data and a command on the basis of the clock, the received clock must be distributed to input terminals for the address, the data and the command and a delay required for transmitting the clock limits the performance of the chip. Also when controlling an output buffer on the basis of the clock, the output is delayed by the clock skew to degrade the margin of output data received by an external device.
Following the increase of the operating speed of the semiconductor memory device, further, the following problem arises in an operation test during a step of fabricating the semiconductor memory device or in advance of shipment of products:
The time required for the test increases following increase of the memory capacity of the semiconductor memory device, to result in increase of the cost for the test as well as the fabrication cost for the products.
In order to prevent such increase the test time following increase of the memory capacity of the semiconductor memory device, a plurality of semiconductor memory devices are generally tested in parallel for improving the test efficiency. However, the aforementioned increase of the memory capacity of the semiconductor memory device results in increase of the bit number of an address signal supplied to the semiconductor memory device, a multi-bit structure of a data input/output interface and the like, and the number of semiconductor memory devices simultaneously testable in parallel is limited due to increase of the number of input pins and input/output pins for control signals in each semiconductor memory device.
In general, the number of chips of semiconductor memory devices simultaneously measurable in a tester depends on the number of pins provided on the tester and that of pins required by the chips, and is generally expressed as follows:
(number of pins provided in tester)/(number of pins required by chips) greater than (number of simultaneously measurable chips)
Assuming that the operating speed of a tester for testing a semiconductor memory device is improved following improvement of the operating speed of the semiconductor memory device itself, further, an extremely high-priced tester is required, to also result in increase of the test cost.
In addition, while a synchronous semiconductor memory device employs complicated systems such as BIST (built-in self test), generation of a clock by DLL (delay locked loop) and the like to reduce the cost and improve the function, it is difficult to externally observe operating states of these circuits.
An object of the present invention is to provide a synchronous semiconductor memory device capable of reducing a test cost by reducing the number of terminals employed in testing thereby increasing the number of chips simultaneously measurable with a single tester and reducing the data rate for output data to be observed thereby allowing efficient testing without employing a high-priced tester of high performance.
Another object of the present invention is to provide a synchronous semiconductor memory device simplifying testing or evaluation of an internal circuit by rendering the state of the internal circuit, whose operation is not directly observable from outside, externally observable through an input/output circuit.
Briefly stated, the present invention is directed to a synchronous semiconductor memory device comprising a memory array, a read circuit, first and second data buses, a first output circuit and a first output node.
The read circuit batch-reads first and second stored data from the memory array in response to an address signal. The first and second data buses receive the first and second stored data respectively. The first output circuit receives the first and second stored data from the first and second data buses, performs different conversions in an normal operation and in a test, holds the converted data and thereafter outputs the same. The first output node receives the output of the first output circuit.
According to another aspect of the present invention, a synchronous semiconductor memory device comprises a memory array, a first match detection circuit, a shift register and a second match detection circuit.
The first match detection circuit receives a plurality of stored data batch-read from the memory array in response to a clock signal and detects a match. The shift register receives an output of the match detection circuit. The shift register includes serially connected first to n-th hold circuits (n: natural number of at least 2) fetching the stored data and outputting held data in response to the clock signal. The second match detection circuit determines whether or not all outputs of the first to n-th hold circuits match with each other.
According to still another aspect of the present invention, a synchronous semiconductor memory device comprises a memory array, a BIST (built-in self test) control circuit and a first terminal.
The BIST control circuit controls execution of a self test for the memory array, supplies an address signal and a command signal to the memory array, and transmits/receives stored data to/from the memory array. In a pretest for testing whether or not the self test is executable, the first terminal outputs a result of the pretest.
According to a further aspect of the present invention, a method of testing a synchronous semiconductor memory device including a memory array, a BIST (built-in self test) control circuit controlling execution of a self test for the memory array, supplying an address signal and a command signal to the memory array and transmitting/receiving stored data and a first terminal outputting, in a pretest for testing whether or not the self test is executable, a result of the pretest comprises a first step and a second step.
The BIST control circuit includes a RAM part storing test data corresponding to the procedure of the self test and a pattern generator part controlling the self test on the basis of the test data stored in the RAM part. The result of the pretest includes the test data stored in the RAM part. The RAM part includes first to n-th groups of storage units (n: natural number) which are units selected by the pattern generator part in execution of the self test. Each group of storage units have m storage units (m: natural number) batch-selected to output the test data to the pattern generator part in execution of the self test while serving as serially connected shift registers in the pretest.
In the first step, the test data is inputted from the first terminal and sequentially shifted and stored in the first to n-th groups of storage units.
In the second step, the test data set in the first to n-th groups of storage units is sequentially shifted and read from the n-th group of storage units through the first terminal.
Accordingly, a principal advantage of the present invention resides in that the number of data input/output terminals necessary for monitoring data output as well as the number of channels employed by a tester for testing the semiconductor memory device can be reduced, whereby the cost for testing the semiconductor memory device can be reduced.
Another advantage of the present invention resides in that non-defectiveness/defectiveness of the synchronous semiconductor memory device can be determined with a small count of observation of a test result output signal, so that a tester of low performance can determine non-defectiveness/defectiveness of the synchronous semiconductor memory device.
Still another advantage of the present invention resides in that the operation of a circuit controlling a self test can be previously confirmed before executing the self test.
A further advantage of the present invention resides in that whether or not the RAM part of the BIST control circuit normally operates is recognizable and the number of pins necessary for testing the RAM can be reduced.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.